Cannot use a type variable absent from the function arguments

The following is an excerpt of a code that was working in the 'old' system. It is not well-typed anymore: Unbound type variable: 'c

  function compose (g:'b -> 'c) (f:'a -> 'b) : 'a -> 'c
  axiom compose_def : forall g:'b -> 'c,f:'a -> 'b,x:'a.
    compose g f x = g (f x)

  function const (x: 'b) : 'a -> 'b
  axiom const_def : forall x:'b,z:'a. const x z = x

  predicate extensionalEqual (f g:'a -> 'b) =
    forall x:'a. f x = g x

  let lemma const_compose_right (f:'a -> 'b) (x:'a) : unit
    ensures { compose f (const x) = (const (f x): 'c -> 'b) }
  =
    assert { extensionalEqual (const (f x) : 'c -> 'b) (compose f (const x)) }